The inventive concept relates to integrated circuits, and more particularly to an active filter using post integration time control and a delta-sigma modulator including the active filter.
An active filter passes or rejects signals having frequencies within a predetermined range or band. In general, an active filter includes not only passive components, such as a resistor, a capacitor, etc., but also one or more active components, such as an operational amplifier. The so-called response characteristic of an active filter may be more easily modified than the response characteristic of an analogous passive filter (i.e., a filter including only passive components). The active filter, as well as circuit elements coupled to the active filter, (e.g., a power supply, a load, etc.), are not affected by each other when the active filter employs an operational amplifier having high input impedance and low output impedance.
A delta-sigma modulator may be employed within an active filter as a loop filter. The delta-sigma modulator is widely used to minimize quantization noise that may be caused when an analog signal is converted into a digital signal. As conventionally implemented, delta-sigma modulators may be classified as a discrete-time delta-sigma modulator and a continuous-time delta-sigma modulator according to the type of loop filter being implemented.
The discrete-time delta-sigma modulator has the advantages of low filter coefficient variation and high frequency scalability. However, the discrete-time delta-sigma modulator has high power consumption, and a switching noise may be generated at its input stage. The discrete-time delta-sigma modulator further requires the inclusion of an anti-aliasing filter to minimize aliasing that occurs when a high-resolution signal is converted into a low-resolution signal.
In contrast, the continuous-time delta-sigma modulator may not require an anti-aliasing filter and consumes less power, as compared with the discrete-time delta-sigma modulator. However, filter characteristics of the continuous-time delta-sigma modulator may be changed depending on a passive component variation due to a manufacturing process variation, an operating temperature variation, or the like.